This image acquisition is conventionally performed using a device fitted with a silver film of large dimensions, typically 60 mm×60 mm or 114 mm×114 mm and up to 230 mm×230 mm, and means for taking a photograph with exposure times between 1/100 and 1/1000 s.
This type of device may be replaced with an electronic device, which is less expensive in terms of operational infrastructure and easier to use. Such an electronic device installed for example onboard an aircraft conventionally comprises a linear array of photodiodes, for example 114 mm in length, placed perpendicular to the direction of movement of the aircraft. The image is obtained by scanning, that is to say during movement of the aircraft, the image being acquired in “push-broom” mode. To obtain an image measuring 114 mm×114 mm, a scanning time of about 10 s is required.
However, the aircraft is subjected to various movements during these 10 s and especially rotational movements perpendicular to the scanning direction (roll), which causes lateral distortion of the image, sometimes making it illegible. This lateral distortion is illustrated schematically in FIG. 1. This figure shows the projection of the linear array 1, which as the scanning proceeds undergoes displacements perpendicular to the scanning direction, the initially intended field 100, which has the shape of a strip, and the recorded image 200 (shaded gray), in which image 200 there are missing regions 300.
One method commonly used to remedy this problem is to stabilize the device, while an image is being taken, by means of optomechanical gyrostabilization of the line of sight. Another method, applied after an image has been taken, is to bring the lines back into mutual register by means of an algorithm so as to obtain an exploitable image, but this consequently has a reduced field of view.